Machines presently used for the production of printing plates by exposing their sensitized surface to light or heat rays generally produced by laser rays are commonly known as platesetters. In these machines a flexible plate is wrapped around the outside or inside of a drum. Devices of this kind require delicate and time consuming operations for introducing, forming and straightening plates as illustrated in U.S. Pat. No. 5,699,740.
Among other machines avoiding the use of a drum, a photocomposer has been constructed as described in U.S. Pat. No. 4,746,942 under the title of “Photocomposing Machine and Method” by same inventor. This machine is more appropriate for film imaging than for printing plates of some rigidity such as metal plates. It includes a laser at a fixed location remote from the plate, necessitating complex optical means that limit the size of the imaging area. In addition, the driving of the photosensitive material by pinch rollers located at a certain distance from the imaging head has the general problems associated with the use of pinch rollers for wide material. It is difficult to adjust and does not insure accurate displacement of both margins of the film and exact repeatability.
Another flat bed scanner described in U.S. Pat. No. 4,764,815 involves the use of a complex pneumatic double-platen system to carry and step the media to be scanned. The reciprocating head mechanism includes inertia operated springs, and a complex and expensive hard to control acousto-optic deflector unit located at a fixed position with mirror means to transfer light beams to the reciprocating head with no focusing means for imaging.
Among flat bed machines, there are also photoplotters based on the use of interlaced light emitting diodes located on an imaging head above a photosensitive plate fixed to a heavy table. In these machines, a considerable mass must be moved and the illumination they provide is not adequate for the sensitivity of metal plates and for high production rates.
Another device based on essentially the same configuration of a projection head moving in two perpendicular directions over a fixed plate is described in GB-A-2 299 728 (Neilson et al.). This particular device scans a light beam across the surface of a photosensitive plate attached to a bed. It comprises a bridge that extends across the bed from one side to the other, supported at each end by independent carriages movable along rails located on the side of the bed perpendicular to the bridge. A scanning head can move along guides extending the length of the bridge. Drive means and position location means are located at each end of the bridge and control means monitor the operation of each drive means in accordance with position information. The device is located above a storage for plates and means are provided for selecting plates of the desired size and feeding them upwards and to a layer of air at a location defined by stops.
Other flat bed devices use a laser beam deviated by a rotating mirror associated with a continuously moving film or plate. The scanning length of these deflection devices is limited. The use of several adjacent deflection systems could be utilized to extend the scanning range but at the cost of expensive and difficult beam junction means.
For the exposition of thermosensitive printing plates, the machines using a deflection device apply a YAG laser pumped by thermostabilized laser diodes and an acoustic or optical modulator. Besides the high costs, these devices are limited in power and modulation frequency. They do not satisfy the purposes of the present invention.
The production speed is not important in imaging machines outside the field of the present invention. For example U.S. Pat. No. 4,746,942 describes a machine for imaging substrates for the electronic industry where output quality is primordial and speed secondary. According to this document the substrate-supporting table weighs six times more than the printing plate and its moving mechanism of the present invention. This difference that could be translated in terms of productivity precludes the use of this device for the printing industry.
Multibeam devices where each beam is produced by an individually controlled emitter do not show these limitations. However, the number of emitters is limited for economic considerations, for example to 64. To achieve the desired performance, the radiation-sensitive support (printing plate) should move at more than 5 meters/second. This speed can only be obtained with machines where the support must be wrapped around the outside or inside of a drum.
In a flat bed device where a flat printing plate of some rigidity should not be deformed, several hundreds of beams are necessary and the best, if not the only, solution is the use of a spatial modulator as described for example in U.S. Pat. No. 4,746,942 (Moulin). In this document the laser and modulation components were at a fix location remote from the device not to overload the moving optics and adversely affecting the speed of the machine.
The inventor and his team in the years following the filing of the patent referred to above have been able, through innovation and taking advantage of present-day available components, to reduce the size and weight of the multibeam source, so that it can be attached to a fast-moving scanning head.
The power outputted by the head and the number of spots (256) make it possible to conceive a flat bed device operating in a start-stop fashion. In this concept, the support must be moved and stopped accurately and quickly during the direction reversal of the optical head. In order to insure desirable image quality, the distance between the support and the projection lens must be accurate within 50 microns. This result could be obtained by firmly attaching the radiation sensitive support to the accurately flat surface of a table moving in a start-stop fashion. This solution is unacceptable in a machine that should produce printing plates at a high rate (for example for the newspaper industry) for it would take too long for the table to be moved and stabilized because of its inertia.